Describe code in its own terms, not by an earlier build's data (closes #522)

Several comments justified code by what an earlier build would have
left in a database or on disk. Each now says what the code does:
reserveSQLiteFile sets the mode on every open whoever created the
file, and expired treats a session lacking a deadline's timestamp as
expired. The comments on retention, resubmitted events, archive
pruning, target headers, the redactor and the delivery result view
drop their mentions of older rows. parseMaxRetries loses its
paragraph on targets saved before the ceiling existed, and neither
BeforeSave nor retentionCutoff cites a migration. Comments only.

Model: opus-5-5

internal/delivery/target_redact.go

@@ -13,9 +13,9 @@ import (
 const RedactionMarker = "(redacted)"
 
 // Redactor removes one target's own credential material from
-// text that target's remote peer chose: a delivery response
-// body, or a delivery error stored before the delivery path
-// learned to mask the URLs it embeds.
+// a delivery response body, whose text the target's remote
+// peer chose, and from a stored delivery error as a second
+// line behind the delivery path's own URL masking.
 //
 // It removes byte-identical echoes of strings taken from the
 // target's stored configuration, and nothing else. Anything